The present invention relates to machinable components for downhole drilling tools. More particularly, the present invention relates to a machinable component for a downhole drilling tool that maintains its structural integrity when exposed to high pressure environments.
Downhole operations, such as those performed in the drilling and/or production of hydrocarbons, are typically performed at extreme depths and at extremely high pressures and temperatures. Such conditions can cause difficulty in performing downhole operations, and often cause damage to wellbore equipment. It is, therefore, necessary that downhole equipment be capable of performing under such difficult conditions.
Downhole drilling tools are subject to external downhole pressures generated by the wellbore and surrounding formations. Additionally, these drilling tools are exposed to internal pressures resulting from high pressure drilling fluids that are pumped through the downhole tool during drilling operations. High pressure drilling fluid is circulated from the surface down through the drilling tool and to the drill bit. The fluid travels through the drill bit and returns to the surface carrying cuttings from the formation.
FIG. 1 illustrates a conventional drilling rig and drill string. Land-based rig 180 is positioned over wellbore 110 penetrating subsurface formation F. The wellbore 110 is formed by rotary drilling in a manner that is well known. Drill string 190 is suspended within wellbore 110 and includes drill bit 170 at its lower end.
Drill string 190 further includes a bottom hole assembly, generally referred to as BHA 150. The BHA may include various modules or devices with capabilities, such as measuring, processing, storing information, and communicating with the surface, as more fully described in U.S. Pat. No. 6,230,557 assigned to the assignee of the present invention, the entire contents of which are incorporated herein by reference. As shown in FIG. 1, BHA 150 is provided with stabilizer blades 195 extending radially therefrom.
The drilling string has an open internal channel 100 through which the high pressure drilling fluid/mud 120 flows from the surface, through the drillstring and out through the drill bit. Drilling fluid or mud 120 is pumped by pump 140 through the internal channel 100, inducing the drilling fluid to flow downwardly through drill string 190. The drilling fluid exits drill string 190 via ports in drill bit 170, and then circulates upwardly through the annular space 130 between the outside of the drill string and the wall of the wellbore as indicated by the arrows. In this manner, the drilling fluid lubricates drill bit 170 and carries formation cuttings up to the surface as it is returned to the surface for recirculation.
The mud column in drillstring 190 may also serve as the transmission medium for carrying signals containing downhole parameter measurements to the surface. This signal transmission is accomplished by the well-known technique of mud pulse generation whereby pressure pulses are generated in the mud column in drillstring 190 representative of sensed parameters down in the well. The drilling parameters are sensed by instruments mounted in the BHA 150 near or adjacent to the drill bit. Pressure pulses are established in the mud stream within drillstring 190, and these pressure pulses are received by a pressure transducer and then transmitted to a signal receiving unit which may record, display and/or perform computations on the signals to provide information of various conditions down the well.
Due to the harsh conditions for downhole operations, the design of downhole pressure housings is typically dictated by the strength required to withstand the high pressure, high temperature and shock conditions of the drilling process. In the assembly structure design process, materials are typically selected based on the loading requirements, which include high pressure, axial compression, and the material weakness as a result of temperature, bending, and shock during the drilling process. High strength materials may be used for these high-pressure applications. Unfortunately, these materials will have a low machinability when compared to conventional materials such as regular stainless steel.
During drilling operations, it is common for the down hole assembly to be in an environment where the outside diameter of the tool is exposed to low pressure and the internal portions of the tool (particularly where the drilling fluid flows) are exposed to high pressure. Therefore, it is necessary to design a structure that maintains both its internal and external integrity when simultaneously exposed to different pressures during drilling operations. One solution would be to select a single high strength material of a given thickness for this application. However, in addition to the necessity for the tool to be able to withstand these drilling pressures, the tool may also support potentially delicate instruments, such as circuit boards used in measurement while drilling (MWD) operations. The process of installing such instruments involves complicated machining operations. During the component mounting process, it may be necessary to create deep milled pockets within the downhole tool and drill threaded holes in order to adequately secure all of the components. In a typical MWD component, it may be necessary to drill hundreds of holes to secure the circuit boards.
As a result of the various conditions under which the tool must operate and the internal design of the tool, there are some conflicting requirements for the construction of this tool. The drilling tool is provided with an internal pressure housing or chassis removably positioned within the drill collar or BHA. In order to withstand the environmental loading, it is typically necessary to use a high strength material to form the chassis. However, high strength materials typically have a low machinability because surface hardness is proportional to strength. Materials that are more amenable to machining may not have the required strength to withstand the high pressures encountered during drilling operations. As a result of the high-pressure environment in which the tool will operate, the common practice is to use low machinable superalloy materials and endure time consuming and/or low efficiency machining processes in order to create the mounting surfaces for the instruments.
Although high strength alloys are necessary for use in high-pressure environments, as previously mentioned, these alloys also take longer to machine. This longer machining time is often the result of reducing the feed rate and turning speeds while machining high surface hardness materials, in order to minimize wearing and chattering of the cutting tools. Using these alloys for parts that require considerable milling and have numerous tapped holes, therefore, adversely affects the manufacturing cost. In addition, during the milling process used to create these pockets in the chassis of the downhole tool, the material is machined down a required depth needed to mount the instrument such that it can properly fit in the chassis. However, the chassis usually must maintain a minimum thickness, and, therefore, a maximium machining depth. The design requires a minimum internal thickness of the material in order to assure maximum strength against the high pressures of the drilling fluid. If during machining, this minimum thickness is exceeded, it may be necessary to scrap the entire chassis part and begin the entire machining process again. Also, if there are mistakes during the machining operation, the part may be scrapped because subsequent repairs typically affect the integrity of the chassis.
A review of the implementation of a tool in a downhole environment indicates that stress is not uniformly distributed through the cross-section of the downhole tool. As a result, high strength (or high yielding) material is not required through the entire cross section of the chassis. In fact, material located beyond a calculated internal diameter from the surface exposed to high pressure can have a lower yield strength and still provide enough structural support to function reliably. Manufacturing a raw material that has the optimal properties located through the cross section can reduce cost and add design flexibility without affecting reliability. Since different portions of the chassis are exposed to various pressures, one alternative could be to construct the chassis from multiple metals based on the pressure and machining requirements.
Various techniques have been developed for providing materials exposed to harsh environments. For example, U.S. Pat. No. 6,309,762 issued to Speckard describes an article of manufacture with a wear resistant cylindrical surface positioned in a channel therethrough, and U.S. Pat. No. 4,544,523 issued to McCullough et al. describes a method of producing an alloy article by compacting metal particles along an internal channel thereof. Another example involving a surface oil field operation is U.S. Pat. No. 6,148,866 issued to Quigley et al. Quigley teaches a spoolable composite tube formed of polymer-based materials for use in high strength tubes that act as pressure housings. In these examples, components are not mounted to the surfaces of the wear resistant or high strength materials. Additionally, these tubes are not designed to take full differential pressure, but only the pressure difference between the annulus and the ID of the tube.
Despite the development of such techniques for dealing with harsh conditions, there remains a need to provide materials capable of enduring downhole conditions while reducing the difficulties encountered in the manufacture and/or machining process.
For downhole drilling operations, not every location along the downhole assembly chassis is exposed to the same pressures during drilling operations. In fact, the outer surface of a chassis, which requires machining in order to mount the instruments, is only exposed to atmospheric pressure. Typically, the internal structure of the assembly, where the drilling fluid flows and which has reduced machining requirements, is exposed to the high pressures. Accordingly, there remains a need for a BHA that can be constructed with material(s) capable of withstanding the environmental loading, but also having high machinability. It is desirable that such a tool be more easily manufactured, more easily maintained, have reduced wear on the tools used to machine the assembly, and extend the life of the manufacturing equipment (mills, taps, etc.) It is also desirable that the tool provide one or more of the following benefits, among others: endurance in even high pressure drilling operations, compatibility with drilling fluids resistance to pressure, ease of manufacture and/or assembly, ease to repair, and resistance to erosion.